In a digital printer or copier, or in any situation in which sheets are transported through an apparatus and accumulated in a stack, the effects of static electricity must be taken into account. The charging events associated with xerography, or even just the sliding contact of sheets against structures within a machine, cause individual sheets to have static charges. When such charged sheets are accumulated in a stack, such as for stapling, the mutual repulsion of like-charged sheets causes the edges of upper sheets on the stack to rise a significant distance from each other, so that the top sheet at any time is not remotely flat. The raised edges, of course, interfere with subsequent activities such as stapling or collating.
A generally-known approach to this problem is to discharge each sheet as the sheet approaches the stack. The discharging is typically done by having the sheet contact a substantially grounded brush or other member as it moves toward the stack, thereby discharging the sheet. U.S. Pat. Nos. 2,883,190 and 5,123,893 show typical ways of applying a discharging brush to a moving sheet.
The use of a “static brush” contacting individual sheets directed toward a stack has some disadvantages, such as possible image area contamination, and does not necessarily fully address discharge of the accumulated additive charge of a thick stack of sheets having small retained charges, especially in dry ambient conditions.